Is there any tutorial on creating a slideshow with transition and with images of different sizes all done from python script?
Based on a concept tutorial that i made there is this addon from hapit at blenderartists forums http://blenderartists.org/forum/showthread.php?360518-Addon-Dynamic-Slideshow basically you use the inport images as planes then place an animated camera over each image. This animation is editable per camera and occurs simultaneously in the scene.  Then in the vse each camera gets its own strip to mix as tou like. This allows you to rearrange the order of playback easily.

The main point here is retaining the full image resolution which the vse cannot do as it always conforms source to project dimensions or crops at 1:1.